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Thursday, 17 October 2013

Water and the composition of Martian magmas

This article has been written by J. Brian Balta and Harry Y. McSween Jr. affiliated to University of Tennessee - USA.

Shergottites, the most abundant martian meteorites, represent the best
source of information about Mars’ mantle and its dissolved
water. If the mantle was wet, magmatic degassing
could have supplied substantial water to the martian surface early in
its
history. Researchers have attempted to reconstruct
the volatile contents of shergottite parental magmas, with recent
analyses
confirming that the shergottites contained
significant water. However, water is not a passive tracer; it directly
affects
magma chemistry and physical properties.
Deciphering the history of water on Mars requires understanding how that
water affected
the chemistry of the shergottites and how they fit
within Mars’ geologic history. Both topics present difficulties, as no
shergottite-like rock has been found in
stratigraphic context and there is debate over the timing of eruptions
of shergottite-like
magmas. Partial melting experiments on terrestrial
basalts and new data from orbiters and rovers on Mars provide the
information
needed to overcome these difficulties and explain
the role of water in shergottite magmas. Here we show that shergottite
compositions
and their martian geologic context can be explained
by melting of an originally wet mantle that degassed over time. We also
demonstrate that models for the evolution of the
martian mantle that do not consider water fail to account for the
shergottite
compositions, surface distributions, and ages.
Finally, we suggest that dehydration of the martian mantle has led to
changes
in magmatic chemistry over time, with shergottites
representing melts of water-bearing mantle and rocks similar to
nakhlites
representing melts of other mantle sources.